Epoxy resins have generally been employed in a variety of applications requiring high performance materials. Catalytic cure of an epoxy resin can often be accomplished in a "one package" system. Preferred such systems have been based on the employment of a lewis acid catalyst in the form of an amine complex such as boron trifluoridemonoethyl amine. The lewis acid is released on heating; cure takes place within 1 to 8 hours and can require a temperature of 160.degree. C. and higher. Due to the temperatures required, these one package epoxy compositions cannot be employed in conjunction with heat sensitive devices. Nor can epoxy monomers having low boiling points be used due to the resulting losses to evaporation during cure. Consequently, improved systems are desirable.
As shown by Schlesinger, U.S. Pat. No. 3,708,296, certain photosensitive aromatic diazonium salts can be employed to cure epoxy resins. When photolyzed, these aromatic diazonium salts are capable of releasing, in situ, a lewis acid catalyst which can initiate the rapid polymerization of the epoxy resin. However, even though these one package epoxy resin mixtures can provide fast curing compositions, a stabilizer must be used to minimize cure in the dark during storage of these mixtures. Despite the measures, gellation of the mixture can occur even in the absence of light. In addition, nitrogen is released during UV-cure, which can result in film imperfections. Diazonium salts are generally unstable, rendering the use of such materials hazardous because of the possibility of run-away decomposition.
U.S. Pat. No. 4,138,255 of Crivello described yet another catalyst system. It employs radiation sensitive aromatic onium salts of the formula: EQU [(R).sub.a (R.sup.1).sub.b (R.sup.2).sub.c X].sub.d.sup.+ [MQ.sub.e ].sup.-(e-f)
wherein R is a monovalent aromatic organic radical; R.sup.1 is a monovalent organic aliphatic radical selected from alkyl, cycloalkyl and substituted alkyl; R.sup.2 is a polyvalent organic radical forming a heterocyclic or fused ring structure selected from aliphatic radicals and aromatic radicals; X is a Group VIa element selected from sulfur, selenium and tellurium; M is a metal or metalloid; Q is a halogen radical; a is a whole number equal to 0 to 3 inclusive; b is a whole number equal to 0 to 2 inclusive; c is a whole number equal to 0 to 1; where the sum of a+b+c is a whole value equal to 3 or the valence of X; d=e-f; f=valence of M and is an integer equal to from 2 to 7, inclusive; and e is &gt;f and is an integer having a value up to 8. These catalysts are ordinarily activated by radiant energy such as an electron beam or ultraviolet light.
Further improvements in this basic Crivello patent are described by the inventor in U.S. Pat. Nos. 4,173,551 and 4,216,288 as well as application Ser. No. 152,424 filed May 22, 1980 now U.S. Pat. No. 4,329,306, issued May 11, 1982. Therein, a cocatalyst such as an organic acid or copper salt is employed with an onium salt. This combination catalyst produces a more stable one package epoxy system at ambient temperature which is nonetheless readily activatable.
In Appln. Ser. No. 176,723 filed Aug. 8, 1980 of Dudgeon, a still further improvement to the basic Crivello patent is described. Therein, onium salts are employed in conjunction with a peroxide compound. This combination again allows thermal activation of a storage stable epoxy system.
The foregoing patents and applications deal chiefly with epoxy systems useful for coatings, laminants, varnishes, sealants and the like. In addition to those uses, epoxy systems which can be utilized as molding compounds are known and are of particular importance. Such molding compounds should be activatable for rapid cure and are especially dependent upon storage stability.